Foibles of Biplanes

[Matt Watts]

Hello again forum! It's been a while...

 

So. I'm a member of a team undertaking the BMFA Heavy Lift Challenge. At present, we still have to decide on a basic design for the project. Given that the wingspan cannot exceed 2m, but the airframe must carry a payload of 4kg, a variety of solutions have been proposed.

 

My personal choice would be to go with a biplane design - however, I understand that biplanes can be... complicated. Doubling the number of wings doesn't simply double available lift, and the weight penalty is not to be taken lightly (I'm sorry, that pun was to intended more to amuse myself more than anyone else).

Currently, I'm looking into ways to reduce the level to which the airflow going past one wing interferes with the airflow going past the other. Apart from staggering the wings and trying to keep the vertical space between them as large as reasonably possible, what can we do to minimise drag and maximise lift? And are there any particular things we should be aware of with biplanes?

 

Oh, and while I'm at it, is designing a biplane a terrible idea? And if so, why?

[Turbycat]

Don't know much about the aerodynamics of biplanes but love flying them. Mine are all WW1 /pre WW2 bipes so they are quite draggy affairs. They do look the best tho'

 

I'm sure you could design a fairly streamlined version.

 

Andy

[Lindsay Todd]

Might be worth considering a sesquiplane layout, some benefts off added lift and minimising drag at least to some extent, its always going to be a balance of lift drag and power.

 

[Reno Racer]

Sesquiplane - Can I recommend the Nieuport 11

[Lee Smalley]

The biplane design has failed nearly every time on the bmfa challenge the extra weight and drag caused by the layout never does well, you would be better putting your efforts into designing a single high lift broad chord low drag wing section.

[Former Member]

[This posting has been removed]

[Biggles' Elder Brother - Moderator]

Lee and Tom make some good points here. For maximum lift with min drag the biplane is not an ideal solution. But if you are keen to go down that root then take a look at a reverse staggered set up - ie upper wing behind the lower not in front. The reasoning behind this is that most of the inter-wing interference is due to downwash off the upper wing interfering with the flow off the back of the lower wing. By reversing the normal direction of stagger you avoid this.

 

But it has to be said I think there are much smarter solutions - you might want to look at some of Burt Rutan's designs for inspiration in terms of high lift at low speed for low drag.

 

BEB

[Lindsay Todd]

Shame on you BEB, knocking the Biplane idea. I suspect the problem with Rutan's designs however are complexity of build in model terms, full size being extensive composite structures and not always easy to translate into simple structures with conventional materials that tend to be used in the challenge.

[Biggles' Elder Brother - Moderator]

Posted by Lindsay Todd on 10/02/2012 23:53:22:

Shame on you BEB, knocking the Biplane idea.

 

Yes - it did "hurt" a little

Posted by Lindsay Todd on 10/02/2012 23:53:22:

I suspect the problem with Rutan's designs however are complexity of build in model terms, full size being extensive composite structures and not always easy to translate into simple structures with conventional materials that tend to be used in the challenge.

 

Very true - but what I have in mind is imitating some of his ideas on wing planform - true they work at their best with very high aspect ratios - and that might not be possible - but I suspect they'd still be very efficient even in shortened form.

Lifting bodies are also certainly worth looking at - as is anything that might give you more lifting area without pushing up the parasitical drag too much (ie the non-lift-related drag) A lifting body combined with some of Rutan's canard ideas might be an interesting avenue of exploration. Just ideas I'm chucking out

 

BEB

 

 

[Matt Watts]

I always did love Rutan's designs...

I mentioned the lifting body idea to the group a few days ago, which was met with a positive reaction. To be honest, anything that adds lift with minimal weight (and effort) increase is a godsend. I suggested leading edge slats on the leading edge, but the group don't want the added complexity - for the same reason, I have thus far refrained from suggesting canards.

Say, what exactly are the aerodynamic advantages of something like the Long-EZ or Quickie? I mean, apart from being exceptionally slippery beasts.

 

Oh, and for the record: if the biplane idea gets shot down in flames, I'm fine with that. At present, I can't really make an informed decision about what design style to take.

 

Thanks for the replies by the way, I wasn't expecting nearly as much help as this!

[Lindsay Todd]

taking a very simplistic view the forward canard will stall before the main wing and can be desirable and can reduce drag; however can have serious disadvantages as well including deep stall characteristics, particularly with pusher layouts and can be sensitive to surface area.

[Matt Watts]

Our head of aerospace suggested that a pusher configuration loses about 20% efficiency in comparison with a puller. I assume a puller with canards isn't the best idea? I can't imagine the wing being too happy if a deep canard deflection suddenly robbed it of fast-moving air from the forward-facing engine.

[Daithi O Buitigh]

BEB - wouldn't 'reverse stagger' be the normal layout for a biplane? Stagger wings are the ones where the top wing is behind the rear (ala DH5) so 'reverse stagger' would put it in the normal position (ala Camel)

 

Just being pedantic really

[Turbycat]

If you want 2 wings something along the lines of Quickie would be brill. See one of those full size things go!

 

Andy

[John Muir]

I believe that Rutan's designs had few aerodynamic advantages and gained most of their performance from the materials and construction techniques employed. He was using composites and ultra smooth surface finishes before most others had cottoned on. Nowadays there are lots of conventional designs produced in composites which can outperform Rutans designs (Diamond and Cirrus come to mind) simply through drag and weight reduction. Pusher props, in general, are less efficient.

 

Isn't the lowest drag, maximum lift configuration a flying wing?

[Simon Chaddock]

Turbycat

The one feature a biplane (or more) can have above all monoplanes is its strength to weight ratio, assuming of course that you brace it correctly.

With careful design a biplane (with twice the wing area) can actually be lighter than the same span monoplane wing.

The drag will be higher from the interference between the wings and from the bracing but can be offset by a slower flying speed.

The stall can be delayed with positive stagger and even more so if the upper wing has less incidence.

 

Whether a multi plane layout has an overall advantage will depend on the particular requirements of the challenge but for maximum lifting ability for a given span and power it is hard to beat.

 

I hope this helps.

[Former Member]

[This posting has been removed]

[Erfolg]

I assume that there is a limit on the power available (engine size)?

 

With respect to Rutan Designs, I am assuming you are thinking of Canards? If so, the amount of information at model size, that is available is limited. Canards do stall, what ever the theoreticians tell you.

 

With respect to negative stagger wing layouts, I thought that they were arranged, conceptually with slightly different incidences, to ensure that both wings do not stall simultaneously.

 

Although not a biplane fan, the arrangement as per Beech staggerwing, and the Sorrell Hipperbipe has to my mind a lot of merit. The merit being that the central upper wing support is straightforward. No silly Cabanes complicating the build, rigging arrangements and ensuring a strong simple, aerodynamically efficient central support.

 

I would have thought that simplicity, ruggedness is very important. The reasons being self evident, that repairs need to be made simple, the simpler the design, the lighter the design potentially can be.

 

In my mind, I would be designing the airframe with the view it will need repair. Are replacement wings allowed. What and how will repairs be permitted. If no repairs at all are permitted, a really rugged design is required, no frills which could be damaged, would be incorporated. In fact I would suggest over design for some aspects. If replacements are permitted, I would be looking to modularize, to permit replacement of whole parts.

 

Also a must with me would be a tricycle undercarriage. They taxi, take off easier than taildraggers. On landing, they do not nose over, propeller strikes are less prevalent. To reduce drag, I would go for thin wheels (drag reduction), largish diameter (makes dealing with uneven surfaces easier).

 

If unlimited power is available, I would not worry about drag, unless there is  a limit placed on the amount of fuel used.

Edited By Erfolg on 11/02/2012 21:19:31

[Myron Beaumont]

What about a micro light type configuration (hang glider wing with all the weight suspended below) .but with a traction prop .Surely one of the smallest,lightest wings supporting a lot of weight ? Wing could even be fabric .Shame the rules are based on span and not wing area otherwise the hang glider configuration would be a definite candidate IMHO

 

[Stefan Hafner]

I did this challenge a few years ago as part of our course at uni, and one of the other teams there had a go at a biplane, but struggled as we were limited to a .40 size engine, not sure if that rule is still in place. The main issue they had was not having enough power, so make sure your engine is running well.

 

I assume its still judged on the total weight to airframe weight ratio? For this I think if you could pull off the biplane and make light it would do well, as the guys have already said with a reverse stagger it'll help. The one that the other team made when i did it was WAY over engineered so was a bit on the heavy side. I think many teams also used the Selig 1223 or similar wing sections, including the biplane, but i think for a biplane a lower drag coefficient will be more important than having the highest possible lift coefficient.

 

If i were to do it again I think I would go down the lifting body route.

[Erfolg]

Hmmm, The challenge involves a weight ratio, relative to what it can lift?

 

No mention of speed?

 

If just weight ratio is an issue, and not speed. then a Biplane will have a lot of merit.

 

If the aspect ratio is kept low, no struts would be necessary, essentially two mono planes wings.

 

With respect to structure, there is often a requirement for the model to be durable. In this case, it needs to be strong enough to last the comp and any incidents in the comp. Perhaps thoughts around a built up balsa frame, for the fuz, with Depron sheeting. With a thought to taping the sheet for repairs, or taking panels of and sticking back on, new sheet, with UHU Por or similar. Wings could be just open structure Solar filmed. If colour on the body is required, just spray with Artist ink, weights nothing.

 

I guess the other issue, if it is to carry a weight, how is it to be carried, as that will have a major bearing on the body design (if that is where it is to be carried).

 

I guess from the students point of view. A list of design needs should be developed by them. Then an additional list of how the objectives can be achieved. etc etc.

 

Seems a tremendous competition.

 

Perhaps the RCM&E could have a similar competition for old thingies, via clubs or even individuals?

[Stefan Hafner]

If i remeber correctly, the weight was a box about 3"x3"X6", but again it may have changed since then, but Matt will be able to tell us. There was nothing about speed, I seem to remember half the marks were from flying and the rest from oral presentations and documentaion and writeups.

 

My thinking was that a biplane using a very lightweight structure and getting a lot of strength from the bracing wires would be the lightest way I could think of (except a well designed lifting body), assuming that the parts can be made light enough, maybe cnc or laser cut??

 

As there were only 2 flights required most teams went for a lightweight but slightly more fragile structure, and hoped it lasted but at the sorts of weights in them in flight a crash usually writes off the plane

[Erfolg]

Stephan

 

I am assuming now, that the nature of the competition has models operating at the extremes of there capability and that the competition continues until the model is effectively written off?

 

In that case, I would avoid the lightweight structure I advocated earlier and now would suggest an EPP/EPO fus. with possibly a Roach pole type boom. Particularly if this can be replaced if broken.

 

I personally see rigging wires as very poor solution, beloved by scale modellers, yet impractical to rig tensions, anticipate stretch if working at model scales.

 

Personally I would be looking at Carbon Fibre main spar, with simple ribs of balsa (again with repair in mind.

 

Whoever suggested a Biplane, is probably right, that is as long as the pretty, pretty approach is eschewed. Functional body is the way. Also low aspect wings, do not seem to have the tip stall issues that high aspect ratio wings have, reaching high angles of attack, before the tips actually stall and of course lower bending moments etc, both flying and crashing.

 

A lot seems to hang on what repairs would be permitted.

 

I acknowledge that my concepts would not immediately be aesthetically attractive, but maybe could be possibly be functionally attractive.

 

Also from what you are indicating, the presentations and documentation are possibly as important. On that basis, they (the students) need to understand the marking scheme for this aspect.and work as hard in those areas.

[Matt Watts]

Ugh... I swear I can hear the gears grinding along this ethernet cable...

 

We're limited to an Irvine 40 engine, and non-flying scoring is done on empty weight. The cargo box is 80*80*200mm. So I'm aware that a biplane design will almost inevitably end up getting us worse scores in weight - I'd just rather not see our final work spread over the flying area through lack of available lift.

 

The only (major) obstacle for a the lifting body design is the fact that we have to stick a forward-facing tennis ball on the plane, which must be sat at 45 degrees to the ground, and must have a 180 degree arc of 'vision' (it's supposed to be an imaginary camera), which can be neither covered, nor obscured by the prop arc. Of course, this will likely interfere with a fuselage-shaped aerofoil. The biplane idea was actually suggested as a way of getting the 'camera' as far out of the way of the tractor prop as possible. Still, I really, really, REALLY want the lifting body idea to be part of the final design.

 

During the flying part of the competition, we aren't allowed to replace anything but the glow plug, undercarriage components and prop. The wording of the rules is a little vague, but I believe repairs are allowed between rounds - can anyone who did this before verify whether this was allowed in past competitions?

Edited By Matt Watts on 12/02/2012 15:55:32

[Stefan Hafner]

Some people were duct-taping their planes back together, but they may well have changed the rules on safety grounds but you could check in the rules here, not sure if it mentions it.

 

The tennis ball thing is new, and a bit of a nuisance, its almost forcing you to go the pusher route for whatever layout you use, but as myron says, the lifting body is really the way to go then.

 

Myron, have to agree 100% that the drawings and report are as important, i remeber spending many long nights working on the drawings, but the highest drwing score made up for all the hard work